An ultrashort optical pulse source is among the key components for realizing ultrahigh speed, large capacity all-optical communication systems and networks, and is of great application value in ultrawide band optical signal sampling, optical storing, ultrahigh speed photon analog-to-digital conversion, and so on. In practice, it is generally required that an ultrashort optical pulse source have characteristics such as small volume, simple structure, high repetition frequency, low jitter, good universality, low cost and high reliability. In telecommunication applications, in order to obtain ultrahigh speed optical transmitters and arrays thereof that are compact in structure and capable of being manufactured in batches, it is also required that an ultrashort optical pulse source be characteristic of being capable of being integrated with other optical devices.
The gain-switched operation of a semiconductor laser is a simple, reliable technique for producing ultrashort optical pulses, which can be conveniently applied to existing commercial semiconductor lasers, and this kind of lasers can output with flexibility an optical pulse signal of the order of picosecond with variable repetition frequency, to adapt to rates for various communication interfaces or upgrading of networks, and to perform ultrafast sampling, waveform monitoring and the like on signals of different data rates. However, a gain-switched laser suffers from the problems of having large timing jitter and frequency chirp. Such problems can be solved by means of light injection locking and optical filtering.
Light injection locking of a gain-switched laser can be categorized as two methods, external light injection and self-seed injection. Although using external light injection technique and self-seed injection technique can achieve wavelength tenability of an optical pulse signal in a gain-switched operating state and the effect of suppressing timing jitter, both of the two techniques have defects. The self-seed injection technique requires that the relationship as defined by the following formula {circle around (1)} be strictly satisfied between the length L of the outer cavity loop of a gain-switched laser and the repetition frequency fr of an ultrashort optical pulse periodic sequence produced thereby:
                              f          r                =                  m          τ                                    1        ⁢        O            where m is a positive integer, T represents the transmission time required for an ultrashort optical pulse signal to pass through the outer cavity loop and is related to the length L. If the transmission speed of the optical pulse signal in the outer cavity loop is v, then there stands
  τ  =            L      v        ❘    .  Once the outer cavity loop is designed, the length L of the outer cavity loop will be fixed, and the repetition frequency fr of the ultrashort optical pulse periodic sequence produced by the self-injection gain-switched laser can be determined by the formula {circle around (1)}, and thus the gain-switched mode will lose the characteristic of being capable of changing the repetition frequency fr of an optical pulse signal with flexibility. In practical applications, due to the influence of change in the ambient temperature and the like, in engineering projects it is difficult to ensure that the repetition frequency fr as determined according to the formula {circle around (1)} is always applicable for data rates for interfaces of communication systems and networks, which greatly limits the application range of the self-injection gain-switched laser. Although this problem can be solved by using the external injection technique, when the external injection technique is used the gain-switched laser generally requires a narrow spectral width wavelength-tunable continuous wave (CW) light source to be additionally used as an externally-injected seed light source, which results in increased cost, complex structure and increased volume of the gain-switched laser.